Seismic Performance of New FRP-Hybrid Bridge Piers

Transcription

1 Seismic Performance of New FRP-Hybrid Bridge Piers Shao-Fei JIANG, Phd, Professor Minjiang Scholar of Fujian Province Fuzhou University, China College of civil engineering, Fuzhou University December 6, 2012

2 Outline 1 Introduction 2 3 Experimental Study Fiber-Element Model 4 Conclusions

3 1. Introduction Earthquake frequently occur. Typical damage modes at bridge piers An crushing failure of pier occur at Wenchuan Earthquake The carbonization of pier due to long-term corrosion

4 1. Introduction Large Residual Deformation It is urgent and necessary to improve the seismic capability and durability of piles.

5 1. Introduction How to reduce the residual displacement and improve the durability of piers remain a challenge in bridge seismic design. Secondary Stiffness And what s Secondary Stiffness?

6 1. Introduction Basic Principles of Secondary Stiffness V Bearing capacity and stiffness after yielding Three materials/structures S K 2 A B C K 1 is initial stiffness K 2 is post-yield stiffness K is unloading stiffness r1 < r2 < r3 O K 1 r1 r2 r3 K D According to Takeda model K = K 1 So the piers with high K 2 values have a lower residual displacement.

7 1. Introduction Didier Pettinga proposes a series of simple approaches to increase the post-yield stiffness of structures: (i) using different reinforcement materials with beneficial features in their stress strain behaviors; (ii) re-designing the section geometry and properties of primary seismic-resisting elements; (iii) introducing a secondary elastic frame to act in parallel with the primary system.

8 1. Introduction To reduce the residual displacement, the paper presents a novel concrete structure of hybrid reinforced concrete-filled fiber reinforced polymer (FRP) tubes bridge piers.

9 1. Introduction--Objective Three new types of composite columns and one conventional column are designed and conducted lowcyclic reversed load tests, and their seismic behaviors are studied. FRP bars HRC columns RC columns FRP tubes CFFT columns FRP bars +FRP tubes HCFFT columns

10 2. Experimental Study FRP Tubes Steel Rebar FRP bars

11 2. Experimental Study Photos of specimens

12 2. Experimental Study Specimen Name Table 1 Design parameters of specimens Column Effective Height (mm) Column Diameter (mm) Longitudinal Reinforcement Stirrup BFRP Bar Fiber Architecture Axial Compression Ratio RC Ф16 8@ HRC Ф16 8@80 6Ф CFFT Ф16 8@80 -- HCFFT Ф16 8@80 6Ф8 3 Layers of Bi-directional 3 Layers of Bi-directional Reinforced Concrete Column (RC Column) Hybrid Reinforced Concrete Column (HRC Column) Concrete-filled Basalt Fiber Reinforced Polymer Tubes Column (CFFT Column) New Hybrid Reinforced Concrete-filled BFRP Tubes Column (HCFFT Column)

13 2. Experimental Study Experimental Setup

14 2. Experimental Study Δ/Δy Number of Cycles In 1st phase, the increment was 1mm. In 2nd phase (After steel rebar yielded), the increment was defined as the times of the displacement ( y). The displacement control sequence consisted of three cycles each of 1 y, 2 y, 3 y, and so on, until the specimen failed.

15 2. Experimental Study-Observation Because CFFT column and HCFFT column are confined by FRP tubes, concrete cracks could not be observed obviously. When the FRP tubes crush around the column base, the composite columns transform into RC column and concrete cracks occur. a) RC Column Concrete Crushing b) HRC Column Cracks of both columns gradually develop and extend to its mid-height. Then, the covers of concrete begin to spall off around the column base as the specimens reach their peak strength. Finally, steel bars yield. c) CFFT Column d) HCFFT Column BFRP Tubes Damage

16 Lateral Load (kn) 2. Experimental Study- Hysteretic Curves Drift Ratio (%) PULL PUSH Post-stiffness of RC column 30 0 Axial Load -30 Lateral Load Lateral Displacement (mm) RC Column

17 Lateral Load (kn) 2. Experimental Study- Hysteretic Curves 60 Drift Ratio (%) PULL PUSH Post-stiffness of HRC column Axial Load -30 Lateral Load Lateral Displacement (mm) HRC Column

18 Lateral Load (kn) 2. Experimental Study- Hysteretic Curves 60 Drift Ratio (%) PULL PUSH Post-stiffness of CFFT column 2.67 a b Bearing capacity but drops quickly 5.34 FRP tube failure 30 0 Axial Load -30 Lateral Load Lateral Displacement (mm) CFFT Column

19 Lateral Load (kn) 2. Experimental Study- Hysteretic Curves Residual displacement Ductility Drift Ratio (%) PULL PUSH a FRP tube failure Post-stiffness of HCFFT column b 0 Axial Load -30 Lateral Load Lateral Displacement (mm) HCFFT Column

20 Lateral Load(kN) 2. Experimental Study- Skeleton Curves 60 FRP tube failure Displacement(mm) Skeleton curves of specimens RC HRC CFFT HCFFT

21 2. Experimental Study- Skeleton Curves V Peak Yield point:the Limit Limit point: position :drop first yielding drop to of 85% max to of 85% of load longitudinal the of load in peak skeleton of load FRP steel curves tubes bars failure V V p V u V y K 2 V a K K p 3 3 K V 2 b u V y K 1 K 1 y p u a) RC Column and HRC Column b) CFFT Column and CFFT Column Typical skeleton curves are simplified as tri-linear model including the elastic, strengthen and degradation stages. y p u

22 2. Experimental Study- Skeleton Curves Table 2 Characteristic Values of Test V Curves Specimen name y (mm) V y (kn) p (mm) V p (kn) u (mm) V u (kn) RC HRC CFFT HCFFT Yield load similar Peak load of composite columns increase by 11.8% 16.9% and 15.0%,compared to RC column

23 2. Experimental Study- Skeleton Curves Table 6 Stiffness values of specimens Specimen K 1 (kn/mm) K 2 (kn/mm) K 3 (kn/mm) K 2 / K 1 K 3 / K 1 RC HRC CFFT HCFFT Bilinear factor r of composite columns increase by K 267% 3 /K 1 of 250% HCFFT and column is only 70% 350%,compared of RC to column. RC column.

24 Bearing Capacity Attenuation Ratio 2. Experimental Study- Skeleton Curves RC column, HRC column and CFFT column degrade quickly, especially CFFT column suddenly drops due to the failure of FRP tube. HCFFT column is almost minimum of all specimens. V 0.6 = i Vp Drift Ratio (%) RC HRC CFFT HCFFT is used to evaluate the reduction ratio of bearing capacity.

25 2. Experimental Study-Residual Displacement Table 4 Residual Displacement of Specimens Residual displacement Load stage(mm) RC(mm) HRC(mm) CFFT(mm) HCFFT(mm) Residual Displacement of composite columns decrease by 19.3%, 3% and 20.3% respectively

26 The residual displacement ratio(%) Lateral Load(kN) 2. Experimental Study-Residual Displacement 6 60 RC HRC CFFT HCFFT RC HRC CFFT HCFFT Lateral displacement ratio(%) 0 rhcfft rhrc rcfft rrc displacement(mm) r = H Residual drift ratio is used to evaluate the recoverability of columns.

27 Ductility Cofficient Equivalent viscous damping coefficient (h e ) 2. Experimental Study-Ductility and Energy Dissipation 8 Forward Direction RC HRC CFFT HCFFT 0 RC HRC CFFT HCFFT 0.0 Specimen Name Drift Ratio (%) Single cycle energy dissipation of HCFFT column is less than RC column due to the smaller Ductility residual coefficient displacement. of composite But its columns cumulative energy dissipation is better than the respectively increase by 40%, 74% and other three test specimens due to better ductility 84% than RC column

28 3. Fiber-Element Model element section fiber

29 Lateral Load(kN) 3. Fiber-Element Model Test curves Simulated curves Displacement(mm) RC Column

30 Lateral Load(kN) 3. Fiber-Element Model Displacement(mm) HRC Column Test curves simulated curves

31 Lateral Load(kN) 3. Fiber-Element Model 80 FRP tube damage Displacement(mm) CFFT Column test curves simulated curves

32 Lateral Load(kN) 3. Fiber-Element Model FRP tube failure Displacement(mm) HCFFT Column test curves simulated curves

33 Sufficient Accuracy

34 Parameter Analysis Selection of the layout of the steel rebar and FRP bars 1 Type 1: Radial direction FRP bars Steel rebar

35 2

36 Lateral Load(kN) displacement (mm) FRP and steel bars is cross layout, Residual displacement is the lowest.

37 4

38 Lateral Load(kN) Displacement (mm) Based on the above analysis and easyconstruction Hysteretic, the layout Curves of steel of and 3 almost FRP bars, Type agree 3, is the with best these choice. of 4 and 5

39 4. Conclusions After the piers reach to the peak capacity, CFFT column gives alarming due to the failure of FRP tubes, and it has the minimum reduction in the bearing capacity.

40 4. Conclusions The ductility coefficient of HCFFT column increases by 84% than that of RC column; although the degradation stiffness is 70% of RC pile, the residual displacement is reduced 20% than that of RC column. HCFFT column shows stable bearing capacity, good ductility and repairable performance.

41 4. Conclusions A precise numerical model is established and validated by experimental results. Using the numerical model, parameter analysis is conducted and the layout of steel rebar and FRP bars are selected. More numerical and theoretical analyses are undertaken.

42 2012/12/13 42